DESCRIPTION: Oxidative phenolic coupling, either by a homolytic of heterolytic mechanism, is of great importance in natural products chemistry. Many heterocyclic products, including alkaloids, cyclic peptides, thyroid hormones, and glycopeptide antibiotics are biosynthesized via enzyme-catalyzed oxidative coupling. Our long-range objectives are to gain a better understanding of the mechanism of action of the enzymes that accomplish these transformation and to utilize them for synthetic applications. Although the mechanism of biosynthesis of thyroid hormones has been investigated from some 50 years, the intermediates and the molecular mechanism of the coupling of the two iodotyrosyl residues to form thyroxine are not known. The coupling of two diiodotyrosyl residues in thyroglobulin to form thyroxine (T4) is uniquely different from the usual dimerization reactions characteristic of phenoxy radicals. Our specific aims are: 1) To define the intermediates and the "lost side chain" formed during biosynthesis and to deduce the mechanism of this important and fascinating intramolecular oxidative coupling and rearrangement reaction. 2) To characterize the putative reactive intermediate in the intermolecular coupling mechanism that reacts readily with 3,5-diiodotyrosine residues of thyroglobulin to form T4. 3) To establish the exact role of this reactive intermediate in the overall mechanistic scheme of T4 biosynthesis, 4) To develop a concise one-pot chemical synthesis of thyroid hormones, triiodothyronine and thyroxine. As a diverse array of biologically active cyclic peptides are formed by post- translational oxidative coupling reactions, we intend to investigate the enzymatic mechanisms of C-0 and C-C coupling of aromatic amino acid residues (tyrosine, hydroxyphenylglycine, and tryptophan) in peptides with special reference to the biosynthesis of the cyclic peptides such as vancomycin, chloropeptin 1, complestatin, and the bastadins.